Small areas of damage to the cartilage in the knee joint have a limited ability to fill in with fibrocartilage. Complete and normal healing is not likely. Areas of full-thickness lesions larger than two centimeters by two centimeters (two centimeters squared) are not likely to heal without treatment.
Treatment today involves several techniques designed to stimulate bone marrow to make a clot. With multiple fibrin clots, a small defect in the cartilage can fill in and replace normal cartilage with similar cartilage.
One of those techniques is called microfracture. Another is autologous chondrocyte implantation (ACI). The microfracture procedure is done in one step. Tiny holes are drilled around and through the cartilage defect. Blood and fat cells from the bone marrow migrate into the area of the lesion and begin a healing process.
ACI is a two-stage operation. Healthy cartilage is harvested from the patient and placed in a special solution. After several hours, the cells are washed and mixed with a small amount of the patient's blood serum. This is allowed to sit in the lab for several weeks while the cells multiply.
When the cells have increased 20 to 30 times, then the new chondrocytes are injected into the lesion. Special steps are taken to prepare the damaged area first before implantation.
These techniques are fairly new. No long-term studies have been done to show what happens years after the surgery is done. Short- to mid-term studies show good results. Some surgeons prefer the microfracture approach to ACI because of the difference in cost, technical difficulty, and extra surgery of ACI.